First, people like boring in filesystems, and ext4 was seen as a natural and less risky successor to ext3, which most people ran. And I think that's a reasonable perception.
Second, ext4 went out of its way to solve some of the system-level issues that tended to cause problems, before it went into production. For instance, the ext4 developers introduced hacks to deal with software that handles write atomicity incorrectly, making it much less likely that you'd end up with a zero-length file if you cut power or crashed at the wrong time. XFS and JFS had similar issues that actually hit users, breaking invalid-but-widespread assumptions about filesystem semantics. ext4 at first considered having similar semantics, but instead worked around how many programs actually wrote files, which made it safer in practice.
(Also, "in the age of ZFS"? ZFS is by no means the dominant filesystem, or even an obvious contender for that title; it has a few popular features, but it's hamstrung by its choice of license.)
Also, you could upgrade ext3 to ext4 in-place. That was a powerful feature for a distro that had ext3 as the default, or for companies that had a lot of data on ext3 volumes.
First, people like boring in filesystems, and ext4 was seen as a natural and less risky successor to ext3, which most people ran. And I think that's a reasonable perception.
Second, ext4 went out of its way to solve some of the system-level issues that tended to cause problems, before it went into production. For instance, the ext4 developers introduced hacks to deal with software that handles write atomicity incorrectly, making it much less likely that you'd end up with a zero-length file if you cut power or crashed at the wrong time. XFS and JFS had similar issues that actually hit users, breaking invalid-but-widespread assumptions about filesystem semantics. ext4 at first considered having similar semantics, but instead worked around how many programs actually wrote files, which made it safer in practice.
(Also, "in the age of ZFS"? ZFS is by no means the dominant filesystem, or even an obvious contender for that title; it has a few popular features, but it's hamstrung by its choice of license.)